A stent graft typically includes radially expandable stents, a plurality of annular stent rings forming a cylindrical reinforcement structure, and a cylindrically shaped graft material to which the stent rings are coupled. Stent grafts are well known to be used in acting as reinforced grafts which are delivered by a catheter and deployed to bypass a damaged diseased portion of an arterial body vessel (lumen).
At deployment, after insertion and transluminal transport to the point of use within a damaged or diseased body vessel, e.g., an aneurysmal location in an artery, the stent graft is radially expanded. A stent graft is typically self-expanding or balloon expandable by application of pressure applied outwardly from the interior of the stent graft. After deployment, the ends of the stent graft seal against healthy portions of the body vessel to provide a seal upstream and downstream of the damaged (diseased) portion being bypassed.
To avoid leakage around the seal between the stent graft and vascular wall at the stent graft and to avoid other complications associated with migration of a stent graft, it is important to assure that stent grafts maintained fixation within a body vessel at the location of initial deployment. Two basic approaches have been developed to improve fixation of stent grafts at the location of initial deployment.
A first approach to stent graft fixation, sometimes referred to as passive fixation, relies on providing the stent graft with an outward biasing radial force at the contact interface between the stent graft and the interior wall of the body vessel in which it was deployed. Typically, a radial force, biasing toward the interior wall of the body vessel, is supplied by a spring attachment element at one or both ends of the tubular stent graft. The spring attachment element urges the stent graft into abutting contact with the interior wall of the body vessel where frictional forces between the spring attachment element and the vessel interior wall provide a substantial portion of the forces needed to create both a liquid-tight seal between the stent graft and the body vessel and fixation of the stent graft at its location of initial deployment.
However, with this first approach, the radial biasing force provided by the spring attachment element has to be carefully selected. A body vessel's capacity to resist a continuous outward radial force is limited. Excessive outward radial force on the interior wall of a body vessel could cause excess bulging and damage to the body vessel. Also, in cases where the contact interface, sometimes called the landing zone, between the stent graft and the body vessel is small (short), the surface area on the interior of the body vessel available to resist outward radial force of the stent graft may be insufficient to firmly and permanently affix the stent graft. An abdominal aortic aneurysm with a short neck landing zone is an example where insufficient area for application of radial force may be available to affix a stent graft.
A second approach to stent graft fixation, sometimes called active fixation, relies on providing the stent graft with hooked and/or spiked fixation elements which extend laterally outwardly from the stent graft to mechanically engage and/or partially penetrate the interior wall of the body vessel. Typically, multiple fixation elements are located circumferentially about annular structures attached to one or both ends of the tubular stent graft.
However, with this second approach it is often difficult to contain the laterally extending fixation elements within a conventional flexible sheath, well known to those of skill in the art, used for containing a compressed stent graft prior to deployment. The laterally extending fixation elements can act as impediments to smooth deployment of the stent graft within a body vessel as the exposed fixation elements can drag on the interior wall of the catheter and resist sliding prior to being deployed in the body vessel. Leakage through the stent graft seal with the wall of the body vessel occurs if a good liquid-tight seal between the stent graft and the interior wall of the body vessel is not effected by the fixation elements alone without the outward radial force provided by the spring attachment element of the passive fixation approach. Further, the chance of migration of the stent graft is nearly eliminated by the use of hooks or other lateral protrusions when they land in healthy tissue.
What is needed is a stent graft that provides secure fixation and sealing using both passive and active means. Such a stent graft should have a controlled outward biasing radial force and no or reduced impediments to the smooth endoluminal placement of the stent graft.